1. Field of the Invention
This invention is directed to an automatic underwater acoustic impedance measuring apparatus.
2. Description of the Prior Art
A pulse tube has traditionally been used as the apparatus for impedance measurement commencing with the efforts of Erwin Meyer and Eugen Skudryzk, German professors, to develop underwater acoustic absorbers for the German Navy during World War II. Physically, the pulse tube is a thick walled steel structure filled with water and instrumented with acoustic transducers. A gated sine wave with about five cycles to approximate a plane wave is the acoustic signal that is employed. Some tubes are equipped to provide temperature and hydrostatic pressure control for studying samples subjected to varying environmental stimuli.
Typical dimensions of various pulse tubes are as follows:
______________________________________ Length (feet) 7 42 21 42 Inner Diameter (inches) 2 5.56 2.50 3.40 Wall Thickness (inches) 1 2.22 2.75 2.30 ______________________________________
An acoustic signal is generated at the bottom of the tube to be reflected from the sample, mounted at the top, which is attached to a high impedance backing, or mounted against a layer of gas acting as a low impedance backing or mounted in the center of the tube so that there is a water backing. A portion of the signal is reflected and its phase is shifted by the sample. The percentage of energy reflected is expressed as the reflection coefficient. Acoustic impedance can be obtained from a knowledge of the reflection coefficient and of the phase shift.
The conventional method of measuring impedance in a water filled impedance tube processes the reflected signal by nulling it with a known signal of equal amplitude and opposite phase. This is conventionally manually performed by an operator at discrete frequencies and represents a long and tedious process subject to error and to operator fatigue.
One recent example, U.S. Pat. No. 4,305,295 illustrates a portable apparatus for measuring acoustic impedance in air at the surface of curved sound absorber wherein the apparatus consists of a circular, flexible disc held at a constant distance from a curved absorbing surface by pins or flexible ribs. Sound from a loudspeaker is fed to the center of the discs and allowed to propagate radially in the space between the disc and absorber. Radial arrays of microphones on the disc surface sense sound pressure amplitude and phase, from which impedance is calculated. Such apparatus is used for determining the acoustic properties of absorbing linings as installed in ducts of jet engines. It is also applicable for measuring the acoustic impedance of other absorptive surfaces, including for example, acoustic wall and ceiling panels. Another example is illustrated in U.S. Pat. No. 4,289,143 wherein there is described a method of and apparatus for audiometrically determining the acoustic impedance of a human ear in air.
The measuring of underwater acoustic impedance of various materials and acoustic structures is required in the use of such materials and structures such as absorbers, decouplers, sonar domes, and transducers. The acoustic impedance of a material or of an acoustic structure provides significant information regarding the operating parameters or attributes of the structure or of its components. The interpretation and examination of an impedance locus reveals the magnitude of acoustic absorption, reflection, sonic velocity, elastic moduli, dissipation and resonance frequencies. These qualities are measured for purposes related to material development, for evaluation, for product development and for quality control. Some of the hydroacoustic applications derived from acoustic impedance information are manifested as underwater absorbers, reflectors, transducers, sonar domes, and baffles. Dynamic moduli, useful in non-acoustic and in air acoustic applications, are also determined from acoustic impedance information obtained with the use of an underwater acoustic impedance measuring apparatus.